To make downhole measurements while a borehole is being drilled, measuring-while-drilling (MWD) and/or a logging-while-drilling (LWD) systems are generally known which measure various useful parameters and characteristics such as the inclination and azimuth of the borehole, formation resistivity, and the natural gamma ray emissions from the formations. Signals which are representative of these measurements made downhole are relayed to the surface with a mud pulse telemetry device that controls a valve which interrupts the mud flow and creates encoded pressure pulses inside the drill string. The pulses travel upward through the mud to the surface where they are detected and decoded so that the downhole measurements are available for observation and interpretation at the surface substantially in real time.
In drilling a directional well, it is common practice to employ a downhole drilling motor having a bent housing that provides a small bend angle in the lower portion of the drill string. If the drill string is not rotated, but merely slides downward as the hole is deepened by the bit being rotated only by the motor, the inclination and/or the azimuth of the borehole will gradually change from one value to another on account of the plane defined by the bend angle. Depending upon the "tool face" angle, that is, the compass direction in which the bit is facing as viewed from above, the borehole can be made to curve at a given azimuth or inclination. If rotation of the drill string is superimposed over that of the output shaft of the motor, the bend point will simply orbit around the axis of the borehole so that the bit normally will drill straight ahead at whatever inclination and azimuth have been previously established. The type of drilling motor that is provided with a bent housing usually is referred to as a "steerable system". Thus, various combinations of sliding and rotating drilling procedures can be used to control the borehole trajectory in a manner such that eventually it will proceed to a targeted formation. Stabilizers, a bent sub, and a "kick-pad" also can be used to control the angle build-up rate in sliding drilling, or to ensure the stability of the hole trajectory in the rotating mode.
When the above-mentioned MWD system is used in combination with a drilling motor, the tool is located a substantial distance above the motor and drill bit. Including the length of a non-magnetic spacer collar and other components that typically are connected between the tool and the motor, the MWD tool may be positioned as much as 40-200 feet above the bit, which necessarily means that the tool's measurements are made a substantial distance off-bottom. Although such location is quite adequate for many drilling applications, there are several types of directional wells where it would be highly desirable to make the measurements much closer to the bit.
For example, where a plurality of "long reach" well bores are being drilling from a single offshore platform, each well bore is started out substantially vertically and then curved outward toward a target. After being curved, the well bore is drilled along a long, straight path that is tangent to the curve until it reaches the vicinity of the target. There, the borehole is curved downward and then straightened so that it crosses the formation in either a substantially vertical direction or at a low angle with respect to vertical. In this type of directional well, the bottom section of the hole can be horizontally displaced from the top thereof by many hundreds and even thousands of feet. The drilling of the two curved segments, as well as the extended reach inclined segment, must be carefully monitored and controlled in order that the location where the hole enters the formation is as planned. Near bit measurements would allow early monitoring of various characteristic properties of the drilled formations, and allow correction of improper well bore trajectory. Indeed, without such measurements, it may be necessary to back up and set a cement plug higher in the well bore and then drill on a corrected trajectory.
Another type of borehole where very accurate control over the trajectory of the borehole must be carefully maintained is one whose lower portion extends horizontally within, rather than vertically through, the targeted formation. It has been recognized that horizontal well completions can provide significant increases in hydrocarbon production, particularly in relatively thin formations. To insure proper drainage of the formation, it is important that the well bore stay well within the confines of the upper and lower boundaries of the formation, and not cross either boundary. Moreover, the borehole should extend along a path that optimizes the production of oil rather than the water which typically is found in the lower region of the formation, or gas which typically is found near the top thereof. Care also must be taken that the borehole does not oscillate, or undulate, above and below a generally horizontal path along the center of the formation, which can cause completion problems later on. Such undulations can be the result of over-corrections caused by the measurements of directional parameters not being made near the bit.
In addition to making downhole measurements such as the inclination of the borehole near the bit which enable accurate control over borehole trajectory, it would also be highly desirable to make measurements of certain characteristic properties of the earth formations through which the borehole passes, particularly where such properties can be used in connection with trajectory control. For example, identifying a "marker" formation such as a layer of shale having characteristics that are known from logs of previously drilled wells, and which is known to lie a certain distance above the target formation, can be used to great advantage in selecting where to begin curving the borehole to insure that a certain radius of curvature will indeed place the borehole within the targeted formation. A marker shale, for example, can generally be detected by its relatively high level of natural radioactivity while a marker sandstone formation having a high salt water saturation can be detected by its relatively low electrical resistivity. Once the borehole has been curved so that it extends generally horizontally within the target formation, these same measurements can be used to determine whether the borehole is being drilled too high or too low in the formation. This is because a high gamma ray measurement can be interpreted to mean that the hole is approaching the top of the formation where a shale lies as an overburden, and a low resistivity reading can be interpreted to mean that the borehole is near the bottom of the formation where the pore spaces typically are saturated with water.
The advent of extended reach and horizontally completed wells has provided geological targets that demand increased accuracy in directional drilling procedures. To provide more accurate control, it would be extremely advantageous if the downhole measurements could be made as near to the bit as is practically possible to gain information at the earliest point in time on which trajectory change decisions could be made. However, since the lower section of the drill string is typically crowded with a large number of components such as a drilling motor power section, bent housing, bearing assemblies and one or more stabilizers, the provision of a sensor sub near the bit which houses a number of rather delicate measuring instrumentalities has not yet been accomplished for several reasons. For example, there is the problem of telemetering signals that are representative of such measurements uphole in a practical and reliable way, particularly if a mud pulse telemetry system was used where the pulses would have to pass through the power section (rotor/stator) of a downhole drilling motor.
The present invention is directed to a sensor sub or assembly that is located in the drill string very near to the bit, and which includes various transducers and other means for measuring variables such as inclination of the borehole, the natural gamma ray emission and electrical resistivity of the formations, and variables related to the performance of the mud motor. Signals representative of such measurements are telemetered uphole a relatively short distance to a receiver system that supplies corresponding signals to the MWD tool located above the drilling motor. The receiver system can either be connected to the MWD tool or be an integral part thereof. The MWD tool then relays the information to the surface where it is detected and decoded substantially in real time.
An MWD system disclosed in U.S. Pat. No. 4,698,794 detects the rotation rate of the shaft of a downhole turbine and converts this measurement into a series of high frequency pressure pulses in the mud flow stream inside the collars above the turbine. These pulses are detected by a pressure transducer in an MWD tool located further above the turbine, and the MWD tool then transmits related pressure pulses at a lower frequency to the surface. Although this patent suggests the use of a telemetry system having lower and upper transmission channels, the sensor for detecting the turbine rpm and the means for producing pressure pulses is located near the top of the drilling motor, and thus is a substantial distance above the bottom of the borehole. This patent also fails to teach or suggest any means by which important borehole parameters, or any geological characteristics of the formations, might be measured below the MWD tool.
In light of the above, a general object of the present invention is to provide methods and apparatus for making near-bit measurements that can be used to accurately control the directional drilling of a well bore.
Another object of the present invention is to provide a measuring-while-drilling system where measurements made near the bit are telemetered uphole to another telemetry system which relays signals to the surface that are representative of such measurements.
Still another object of the present invention is to provide a sensor sub of the type described which measures borehole trajectory parameters as well as certain geological formation characteristics which aid in maintaining accurate control over the direction of a well bore so that it can be made to penetrate and remain within a targeted formation.
Yet another object of the present invention is to provide a sensor sub of the type described which measures borehole trajectory parameters and certain geological formation characteristics which aid in maintaining accurate control over the direction of a well bore so that it can be properly curved and then extended within a targeted region of an earth formation.
Another object of the present invention is to provide certain azimuthally focused measurements which are used to ensure proper diagnosis of a change in direction that is needed to correct an improper wellbore trajectory. For example, when the drilling of a horizontal wellbore that extends into a hydrocarbon-bearing sandstone reaches a shale strata, the geological measurements made with the near-bit sensors will detect the transition and can be used to determine whether the well trajectory should be corrected upward or downward since such azimuthally focused measurements will show whether the shale layer is above or below the sandstone layer.
Another object of the present invention is to provide a sensor sub of the type described that measures downhole equipment parameters such as motor shaft RPM which enable a continuous monitor of the drilling process, for example respecting wear of the motor stator, optimum weight-on-bit, and motor torque.
Yet another object of the present invention is to provide a sensor assembly of the type described that measures parameters such as vibration levels that may adversely affect the measurement of other variables such as inclination and lie in a regime which can produce resonant conditions that reduce the useful life of tool string components. Such measurement also can be used in combination with surface pump pressures to analyze reasons for changes in the rates at which the bit penetrated the rocks.